Portable communications systems that transmit high bit rate data have high performance demands. Such high performance systems include Satellite News Gathering (SNG) systems, systems for logging and transmitting data from remote exploration sites, and portable military communication systems. In order to achieve high performance while preventing undue interference to or from other systems, such communication systems generally employ an antenna with a suitably sized parabolic reflector. The most practical and least expensive option for such systems is a single-offset antenna in which the feed support arm (with the feed assembly at the top end) can be removed from the parabolic reflector to enhance portability.
FIG. 1 shows a conventional prior art portable satellite communications terminal unit. The unit has a base 106 that includes means for stabilizing the unit on a surface. The base also houses electronic components for processing incoming and outgoing communications signals. The distal end of an elongate support arm 104, commonly referred to in the art as a “boom,” holds a feed horn 110. Throughout this disclosure and the claims, “distal” refers to structures along the feed arm and support arm at, near, or toward the feed horn; “proximal” refers to structures along the feed arm and support arm at, near, or toward the reflector or base.
The support arm is attached by its proximal end to a parabolic reflector 107, commonly referred to as a “dish.” The support arm is shown in FIG. 1 attached to the top of reflector 107. Although this is typical, it is not uncommon to have the support arm attached to the bottom of the reflector or at other points about the periphery of the reflector.
A feed assembly 101 includes the feed horn 110 aimed at the reflector to collect incoming (down-link) received signals reflected from the reflector and to direct outgoing (up-link) transmitted signals to the reflector. The feed assembly also includes an exposed flexible guide 108 for conducting the transmit signal to the feed horn. A receive line 109 conducts receive signals from the feed assembly to the processing circuitry. As shown, a low noise block (LNB) downconverter 102 is often integrated into the receive signal pathway. FIG. 1 also shows a transmit amplifier 105 as typically attached to the back of the reflector 107.
A transmit filter 103A, when used, is often attached to the support arm as shown and runs generally parallel to the arm. Such a transmit filter is particularly important in cases where a high power transmit amplifier is used to meet the up-link requirements because high power transmit amplifiers typically produce a high amount of noise in the receive frequency band that passes through the receive filter. This noise interferes with the performance of the receiver unless preventive measures are taken. The transmit filter, if properly designed, will pass the transmit frequency band with minimum signal loss while suppressing the noise in the receive frequency band. However, in some of the lower frequency bands such as X-band and C-band, filters having sufficiently high performance are relatively large. Placing such a filter near the feed horn results in bulky, awkward structures that cause problems due to weight loading of the support arm and possibly wind loading caused by a large cross-sectional area. Partial obstruction of the signal radiated from the reflector may also occur. Thus the size of the transmit filter typically requires placing it alongside the feed support arm 104 with appropriate attachments to the arm. However, this arrangement significantly complicates assembly and disassembly of the unit in field conditions.
FIG. 2 shows a typical conventional feed assembly in more detail. As shown, the feed assembly typically includes a feed horn 201, a polarizer 202 (in cases of circular polarization), an ortho-mode transducer (OMT) 203, and LNB 102 with an associated receive filter 204. In some cases the receive filter employed is too large to be incorporated into the feed assembly and, together with the LNB 102, is placed outside the feed assembly.
The OMT separates vertically and horizontally polarized signals in the case of linear polarization. The two signals are physically accessed at two waveguide flanges oriented in different directions, as discussed below.
For circular polarization, either a polarizer 202 is placed between the feed horn 201 and the OMT 203, as shown, or a polarizer incorporating the OMT function is used. The latter category is well represented by septum polarizers. A number of patents can be found for various types of septum polarizers, such as U.S. Pat. No. 6,661,390 to Gau et al.; U.S. Pat. No. 6,507,323 to West et al.; U.S. Pat. No. 6,118,412 to Chen, and U.S. Pat. No. 6,724,277 to Holden et al.
FIG. 3 shows a typical septum polarizer 301 in cross-section. Feed horn 302 is connected to the distal end 301A of the septum polarizer. At the proximal end 301B of the septum polarizer are two ports. For instance, port 303 carries the linearly polarized transmit signal 304, which is gradually converted into a left-hand circularly polarized signal as it progresses along the septum to the distal end 301A. Similarly, a right-hand circularly polarized signal entering the distal end of the polarizer is gradually converted along the septum into a linearly polarized receive signal 306 emerging at port 305. Thus, septum 307 converts circular to linear polarization (or vice versa) and separates the transmit and receive signals at the proximal end, hence the name septum polarizer. For the purpose of comprehending the present invention it is important to note that the septum of the prior art septum polarizer is limited just to the polarizer, because the two signals diverge at the proximal end of the septum polarizer into separate waveguides 308, 309, the axes of which often subtend an angle of 180°, as shown in the figure.
As a general rule, the two ports of a septum polarizer are oriented in different directions, usually opposite each other as shown in FIG. 3. While this conventional design is convenient for physical separation of transmit and receive components, it also contributes to a bulky feed assembly in antennas, particularly those used for the lower microwave bands such as X-band and C-band.
As noted above, the prior art devices have a number of disadvantages and problems, particularly with respect to portable units used in the field. Many of these disadvantages and problems are related to the fact that waveguides are handled separately. As a result the feed assemblies have exposed waveguide adapters, waveguide filters, receive-lines, and bulky opposing polarizer ports. These exposed structures on the end of the support arm produce significant weight and wind loading on the arm. In addition, external transmit filters attached to the support arm increase the complexity and time of assembly and disassembly and increase the risk of damage should the unit be knocked over by wind or other forces.
Although all of these problems have not hitherto been resolved in a single device, there have been ad hoc attempts to resolve some of them. For instance, an attempt to improve the mechanics of the feed support arm is disclosed by Canadian patent 2,424,774 to Russell et al, which describes a portable satellite terminal for Ku-band operation in which the transmit filter is contained within a hollow support arm, rather than using the more conventional placement beside the arm. This arrangement is shown in FIG. 2 in which the hollow arm 104 houses the alternative transmit filter 103B. The support arm connects to either the reflector or the base by a flange or other suitable connector means. This allows the integrated support arm and filters to be attached or removed as a single unit.
Another example of attempts to integrate various functions is shown in U.S. Pat. No. 5,905,474 to Ngai et al. wherein a single, appropriately bent waveguide is used to provide both the signal connection to the feed assembly and mechanical support (i.e. feed support arm). However, Ngai does not disclose integrated waveguides and filters. In U.S. Pat. No. 5,708,447 to Kammer et al., two bent waveguides running in parallel are used in a similar way to achieve a similar result. This approach enables both a transmit and receive function with different polarizations or a dual receive only (or dual transmit only) function with different polarizations. But again, there is no disclosure of integration of the waveguides or the filters, nor of any means for integrating multiple waveguides into a single structure that also includes transmit and/or receive filters.
Finally, there have been attempts to place some of the RE front end electronics into the feed support arm; however, these attempts have so far been limited to small receive components such as mixer/amplifiers and either microstrip or coaxial filters. One example of this approach is U.S. Pat. No. 5,523,768 to Hemmie et al. uses a hollow arm containing a mixer/amplifier and a coaxial filter but no waveguide components.
In view of the functional and structural limitations of the present art, what is needed is a rugged, high performance, high speed portable communications system for transmission and reception of data and/or video communications in which the components of the feed assembly and its support arm are unobtrusively integrated into a single streamlined structure that is free of exposed waveguides and filters and that minimizes weight and wind loading to the support arm.